Anatomical analysis and phytochemical screening of Frangula rupestris (Scop.) Schur (Rhamnaceae)

Frangula rupestris and F. alnus are the only two species of the genus Frangula in the flora of the Balkan Peninsula. Frangula alnus is well-known for anthranoid content, and its stem bark and fruits are widely used as laxatives. Data on anatomy, plant metabolites, and potential use of F. rupestris are scarce. In this work we analysed anatomy of the stem and leaves and performed phytochemical screening of the bark and leaves of F. rupestris. Specific anatomical characteristics of the stem include the presence of large mucilage cavities in the bark and pith, as well as numerous parenchyma cells containing solitary or cluster crystals of calcium oxalate. The majority of leaf epidermal cells are filled with mucilage. In the main leaf vein there is parenchyma with numerous mucilage cavities and solitary or cluster crystals of calcium oxalate. The levels of flavonoids, total phenolics, and tannins in bark and leaves of plants from two localities were determined by spectrophotometric methods, and the results were compared with those obtained for bark of F. alnus. Bark and leaves of F. rupestris contained 2.68-3.03% and 2.22-3.77% total phenolics, 1.70-2.10% and 0.57-1.54% tannins, and 0.12-0.36% and 0.57-0.99% flavonoids, respectively. The conducted HPLC and LC-MS analyses of hydromethanol extracts of bark and leaves of F. rupestris revealed the presence of flavonols. The dominant compound in all extracts was hyperoside and its content, determined by HPLC, ranged from 30.40 to 82.03 mg/g. Swelling indices determined for 0.5 g of plant material of F. rupestris bark and leaves (5.8-11.4 and 5.8-13.8) were higher than that of the bark of F. alnus (4.4) and greater than those of the mucilaginous drugs Althaeae folium and Althaeae radix (4.7-4.8). The high swelling indices of F. rupestris bark and (especially) leaves suggest their potential use as bulk-forming laxatives. In addition, differences in the content of metabolites were observed in plants from different localities.